Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. A pressure regulating valve may then be used to supply the compressed natural gas (CNG) at lower pressures to an engine combustion chamber. Various engine systems may be used with CNG fuels, utilizing various engine technologies and injection technologies that are adapted to the specific physical and chemical properties of CNG fuels. For example, mono-fuel engine systems may be configured to operate only with CNG while multi-fuel systems may be configured to operate with CNG and one or more alternate fuels, such as gasoline or gasoline blend liquid fuels. Therein, the engine may be preferentially operated on CNG to reduce gasoline consumption, while turning to gasoline usage when CNG is unavailable.
However, the inventors herein have recognized a potential issue with such systems. There may be conditions when there is sufficient fuel in the CNG fuel tank but insufficient fuel rail pressure in the CNG fuel line. These conditions may arise, for example, due to a flow restriction in the CNG fuel line as caused by a fuel line freeze up (due to excessive water vapor content in the gaseous fuel leading to ice blockage), plugging of a tank coalescing filter (due to compressor oil filling the filter), the presence of a kink or plug in the fuel line, etc. In response to any of these conditions, the CNG tank may be effectively treated as empty and accordingly mitigating actions may be taken. For example, in mono-fuel systems, an aircharge to the engine may be limited, so as to limit engine power. As another example, in multi-fuel systems, engine operations may be switched to the use of the alternate fuel. For example, in the example bi-fuel system discussed above, engine operations may be shifted to the gasoline fuel. In either case, the result is an inefficient use of CNG, since any remaining CNG in the fuel tank is not being used to operate the engine and propel the vehicle. In addition, the switch to gasoline usage degrades the fuel economy of the vehicle.
The inventors herein have further realized that at least some of the above-mentioned conditions that lead to a drop in CNG fuel rail pressure may be temporary in nature and/or may not be resolved by repair. For example, a flow restriction caused by ice build-up in the fuel-line (e.g., due to high humidity fuel and/or low ambient temperatures) may resolve itself once the ice has melted. In comparison, other conditions may be of a more persistent nature and may require repair and maintenance work. Thus, mitigating actions may vary for different flow restrictions.
In one example, some of the above issues may be at least partly addressed by a method of operating an engine comprising alternating fuel injection to one or more engine cylinders between a first, gaseous fuel from a first fuel tank and a second, liquid fuel from a second fuel tank responsive to each of a first fuel rail pressure and a first fuel tank pressure of the first fuel. In this way, a drop in fuel rail pressure resulting from a temporary flow restriction in the CNG fuel line may be better identified, and CNG usage may be resumed when the flow restriction has cleared.
For example, an engine may be configured to operate on a first gaseous fuel, such as CNG and a second liquid fuel, such as gasoline. In response to a drop in CNG fuel line rail pressure below a threshold pressure, while the CNG fuel tank is sufficiently full, an engine control system may infer a fault (e.g., flow restriction) in the CNG flow delivery system and may immediately switch operation of one or more engine cylinders to the gasoline fuel. After operating on gasoline for a duration, the control system may resume operation of the cylinders with CNG. As one example, the switching may include operating some engine cylinders with CNG while operating the remaining engine cylinders with gasoline fuel for the duration. As another example, the switching may include operating a given cylinder with at least some CNG and at least some gasoline fuel for a number of combustion events.
In one example, the duration for which the cylinders are operated with gasoline fuel may correspond to a duration required to melt and clear a potential ice blockage from the CNG fuel line, under the current ambient temperature conditions. Consequently, a temporary flow restriction due to a fuel freezing condition may be overcome after the duration. If upon return to CNG operation of the cylinders, the CNG fuel rail pressure has returned to or above the threshold pressure, without a fuel tank refilling event occurring in the interim, it may be determined that the underlying flow restriction was temporary in nature (and has since cleared). However, if after the duration, the fuel rail pressure remains insufficient, a more persistent restriction in the CNG fuel line may be determined (e.g., due to a filter plugging condition), and gasoline injection to the engine cylinders may be continued. In addition, based on the nature of the restriction, an appropriate diagnostic code may be set.
In an alternate embodiment, where CNG is the only fuel available in the engine's fuel system, in response to the drop in fuel rail pressure, an airflow to the engine may be temporarily limited for the duration. If the fuel rail pressure returns after the duration, injection of CNG may be resumed. However, if the fuel rail pressure does not return after the duration, the airflow may be further limited and an appropriate diagnostic code may be set.
By performing a mitigating action for a specified duration, temporary flow restrictions in a fuel line may be overcome. By alternating injection of a gasoline fuel with CNG fuel over a duration based on a CNG rail pressure, temporary flow restrictions in the CNG fuel line may be better differentiated from permanent flow restrictions, and accordingly addressed. Specifically, by resuming CNG usage after the temporary flow restriction has been overcome, CNG usage may be increased, gasoline usage may be decreased, and vehicle fuel economy can be improved.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.